1. Field of the Invention
The present invention relates to a hydropneumatic shock absorber, and more particularly to a hydropneumatic, self-pumping, telescopic shock absorber constructed to support a vehicle, as a part of a suspension of the vehicle, and thus to maintain a vehicle at a constant height irrespective of different loading conditions.
2. Description of the Prior Art
Generally, each of the wheels of a vehicle is supported by a suspension, and a shock applied to each wheel is absorbed by a shock absorber of the suspension contracting. Specifically, when a shock absorber contracts, a damping force occurs due to resistance of a fluid filling a cylinder. When a vehicle is traveling on a road having an irregular surface, the shock is softened by the damping force before being transmitted to a vehicle body.
However, the shock absorber also contracts when the number of passengers or the weight of a load changes. This contraction of the shock absorber causes the height of the vehicle to change. Thus, a sufficient amount of contraction of the suspension cannot be secured. As a result, desired levels of riding comfort and driving stability cannot be obtained.
For this reason, hydropneumatic shock absorbers such as the ones disclosed in "Self-Energizing Hydropneumatic Leveling Systems" (Society of Automotive Engineers, INC (SAE), Technical Paper Series 780052) have been developed recently. These shock absorbers are operated to maintain a vehicle at a constant height when the number of passengers or the weight of a load has changed.
A conventional hydropneumatic shock absorber unit is equipped with a high-pressure operating chamber filled with a fluid pressurized by a high-pressure gas, a low-pressure operating chamber filled with a fluid whose pressure is controlled by a low-pressure gas, and an inner cylinder whose interior communicates with the high-pressure operating chamber.
A piston fixed to one end of a piston rod of the hydropneumatic shock absorber is inserted in the interior of the inner cylinder. A pump mechanism for transporting the fluid in the low-pressure operating chamber to the interior of the inner cylinder while the piston slides during the compression of the shock absorber unit, and a mechanism for returning the fluid in the inner cylinder to the low-pressure operating chamber when the piston moves beyond a predetermined position during the extension of the unit are built in the piston.
When the piston moves in a contracting direction beyond the predetermined position, the pressure in the inner cylinder increases as the vehicle travels and the piston slides inward. As a result, the piston is forced toward the piston rod, that is, in a direction in which the hydropneumatic shock absorber extends, due to a difference between areas to which the pressure is applied. When the piston has reached the predetermined position, the pressure of the inner cylinder does not increase even if the piston slides further. When the piston moves in the extending direction beyond the predetermined position, the pressure in the inner cylinder decreases. Thus, the piston, which is held inside the cylinder, is maintained adjacent to the predetermined position.
Therefore, when the height of the vehicle in which the above-mentioned pneumatic shock absorber is mounted decreases as a result of an increase in the number of passengers, the weight of the luggage or the like, the pressure on the piston increase as the vehicle travels. When the height of the vehicle decreases as a result of a decrease in the number of passengers, the weight of luggage or the like, the pressure on the piston decreases until the piston is at the predetermined position. The vehicle is thus maintained at a constant height at which the piston is at the predetermined position.
However, the above described hydropneumatic shock absorber is constructed such that the fluid in the low-pressure chamber is transported to the inner cylinder when the wheel bounces (the piston moves in the contracting direction). That is, the above described hydropneumatic shock absorber is constructed such that the transportation of the fluid for moving the piston in the extending direction is performed while the piston moves in the contracting direction.
As a result, there occurs a great damping force when the wheel bounces. When an upward force is applied by the road to the wheel, or when the load of the vehicle is applied downward, a comparatively large shock is transmitted to the vehicle body as a counter force against the damping force. In this respect, the above described hydropneumatic shock absorber has a disadvantage in that the riding comfort is impaired when the wheel bounces.
Another disadvantage of the above described hydropneumatic shock absorber is that an excessive amount of fluid is transported to the inner cylinder and the high-pressure chamber when the vehicle travels over a rough road characterized by continuous irregularities (that is, when the piston undergoes repeated strokes each having a long duration), thereby impairing the riding comfort.